Phosphofructokinase is the key regulatory enzyme of glycolysis in all mammalian tissue. We are investigating the regulatory mechanisms of the enzymes in erythrocytes, liver and muscle. We have shown that glucagon stimulates the phosphorylation of phosphofructokinase which results in increased sensitivity to inhibition by ATP and citrate. We are currently studying the stoichiometry of phosphorylation and correlated changes in the activity. We are also attempting to separate phospho and dephospho forms of the enzymes from liver and isolate a specific protein kinase(s) for phosphofructokinase. A specific kinase and phosphatase for muscle phosphofructokinase will also be investigated.